"(Samson) suddenly jumped and slapped (Baina) and knocked her off her feet and down the side of the enclosure," Simmons said. "She fell and hit her head. There was tremendous head trauma."

Zoo staffers put forth "a heroic struggle" to save her life, Simmons said. The 3-year-old was even taken to the Nebraska Medical Center for a CAT scan. But Baina died at the hospital about 12:30 p.m.

Simmons said it was the second day since Baina's reintroduction to her family in an enclosure away from public view. Baina, who weighed 60 pounds, had been removed during attempts to get the 400-pound Samson to mate with an older female.

Staffers were stationed around the enclosure observing the group, Simmons said. Everything appeared fine until he suddenly struck her.

Simmons estimated Baina fell about 7 or 8 feet.

"It had to be that she just hit wrong because, ordinarily, (gorillas) would land on their hands and feet and be all right," he said. "It was just one of those fluke things."

Simmons said the zoo observers never had a chance to intervene.

"It was just a flick of his wrist, and she was gone," he said. "This is very unusual, because male gorillas may fight each other but they are very tender toward the females and offspring."

Gorillas, the largest of the primates, are ground-dwelling herbivores native to the forests of Africa.

Baina was born at Hubbard Gorilla Valley at the Omaha zoo on April 8, 2005, to Timu, the world's first test-tube gorilla. When Timu did not show the appropriate motherly instincts and walked away from the baby, zoo staff bottle-fed her as they had done with Timu's firstborn.

When Baina's grandmother, Rosie, showed interest in the baby, the zoo allowed her to become a surrogate mother. Baina's death drops the zoo's gorilla population to 13.

Simmons said it has been 38 years since a gorilla at the Henry Doorly Zoo was killed. In that case, a mother gorilla climbed to the top of the cage and dropped her baby to the floor below.

Staffers will get together to talk about Thursday's episode and evaluate Samson's attitude. Everything happened so fast, Simmons said, that the observers are not sure whether Samson intended to harm Baina.

"The only thing predictable about wild animals," Simmons said, "is their total unpredictability."